This invention relates to an art for reducing costs and preventing degradation of a discharge lamp in a discharge lamp lighting circuit adapted to stabilize start performance by control of temporarily fixing the supply voltage polarity to the discharge lamp for voltage supply control before the discharge lamp is lighted.
The configuration of a lighting circuit of a discharge lamp, such as a metal halide lamp, comprising a DC power supply circuit, a DC-AC conversion circuit, and a starter circuit is known. For example, in the configuration wherein a DCxe2x80x94DC converter is used as a DC power supply circuit and a full-bridge type circuit comprising two pairs of semiconductor switch elements for performing switching control and a driver circuit thereof are used for a DC-AC conversion circuit, the positive-polarity (or negative-polarity) voltage output by the DCxe2x80x94DC converter is converted into rectangular-wave voltage in the full-bridge type circuit, then this voltage is supplied to a discharge lamp.
In order to well light a discharge lamp, a technique of supplying a temporary high voltage (open-circuit voltage or open voltage) to the discharge lamp before the discharge lamp is lighted is known. At this time, preferably the supply voltage polarity to the discharge lamp is temporarily fixed; however, the period of fixing the supply voltage polarity is prolonged in the situation where the discharge lamp does not immediately make the transition to a light state if a high voltage signal (so-called starter pulse) generated by a starter circuit is applied to the discharge lamp. Thus, for example, if a bootstrap system is adopted for driving the semiconductor switch elements making up a DC-AC conversion circuit, the capacitance of a capacitor must be preset to a large value because of the requirement for providing the amount of charges accumulated in the capacitor from a power source (this is a problem), causing an increase in costs.
In order to light another discharge lamp with one discharge lamp already lighted in a lighting circuit wherein lighting control related to a plurality of discharge lamps can be performed by a common circuit, if that discharge lamp does not make the transition to a light state, the discharge lamp already lighted is kept in the state in which the supply voltage polarity remains fixed over a long time. Thus a short life, degradation, and the like is caused by placing a thermal stress on the electrode of the discharge lamp, which is a problem.
The present invention reduces costs and prevents short life and degradation of a discharge lamp by placing a time limit on the duration of the period over which the supply voltage polarity is temporarily fixed by voltage supply control before the discharge lamp is lighted.
According to the invention, there is provided a discharge lamp lighting circuit comprising a DC power supply circuit for outputting a DC voltage and a DC-AC conversion circuit for converting the output voltage of the DC power supply circuit into an AC voltage and then supplying the AC voltage to a discharge lamp. In order to light the discharge lamp, the period of time over which the polarity of the voltage supplied from the DC-AC conversion circuit to the discharge lamp is defined as either positive or negative is provided before the discharge lamp is lighted. A polarity inversion means for inverting the polarity of the voltage if the duration of the period exceeds a predetermined time is provided for alternating the polarity of the voltage supplied to the discharge lamp after the discharge lamp is lighted.
Therefore, according to the invention, the polarity inversion means forcibly inverts the voltage polarity so that the time of fixing the polarity of the voltage supplied to the discharge lamp to one polarity before the discharge lamp is lighted does not continue more than necessary.